Geometry for Elementary School/The Side-Angle-Side congruence theorem
The corresponding material in Euclid's elements can be found on page 33 of Book I, Proposition 4 in Issac Todhunter's 1872 translation, The Elements of Euclid for the Use of Schools and Colleges. |
In this chapter, we will discuss another congruence theorem, this time the Side-Angle-Side theorem. The angle is called the included angle.
The Side-Angle-Side congruence theorem
editGiven two triangles and such that their sides are equal, hence:
- The side equals .
- The side equals .
- The angle equals (These are the angles between the sides).
Then the triangles are congruent and their other angles and sides are equal too. Success!
Proof
editWe will use the method of superposition – we will move one triangle to the other one and we will show that they coincide. We won’t use the construction we learnt to copy a line or a segment but we will move the triangle as whole.